In olefin epoxidation, a feed containing an olefin and an oxygen source is contacted with a catalyst under epoxidation conditions. The olefin is reacted with oxygen to form an olefin oxide. A product mix results that contains olefin oxide and typically unreacted feed and combustion products.
The olefin oxide may be reacted with water to form a 1,2-diol, with an alcohol to form a 1,2-diol ether, or with an amine to form an alkanolamine. Thus, 1,2-diols, 1,2-diol ethers, and alkanolamines may be produced in a multi-step process initially comprising olefin epoxidation and then the conversion of the formed olefin oxide with water, an alcohol, or an amine.
Olefin epoxidation catalysts comprise a silver component, usually with one or more additional elements deposited therewith, on a carrier. Carriers are typically formed of a refractory material, such as alpha-alumina. In general, higher purity alpha-alumina has been found to correlate with better performance. It has also been found for example that the presence of minor amounts of impurities in the carrier such as alkali and/or alkaline earth metals and some forms of silica can have a beneficial effect.
Intuitively it might also be considered that the higher the surface area of the carrier, the greater the area available for deposition of the silver and therefore the more effective the silver deposited thereon. However, this is generally found not to be the case and in modern catalysts the tendency is to use a carrier with a relatively low surface area, for example a surface area of less than 1.3 m2/g, or even less than 1 m2/g.
US 2003/0162984 A1 discloses carriers which have a surface area of at least 1 m2/g. The working examples given show improved initial selectivity and activity of epoxidation catalysts based on carriers having at least 70% of the total pore volume represented by pores with diameters in the range of from 0.2 to 10 μm.
The catalyst performance may be assessed on the basis of selectivity, activity and stability of operation. The selectivity is the fraction of the converted olefin yielding the desired olefin oxide. As the catalyst ages, the fraction of the olefin converted normally decreases with time and to maintain a constant level of olefin oxide production the temperature of the reaction is increased. However this adversely affects the selectivity of the conversion to the desired olefin oxide. In addition, the equipment used can tolerate temperatures only up to a certain level so that it is necessary to terminate the reaction when the reaction temperature would reach a level inappropriate for the reactor. Thus the longer the selectivity can be maintained at a high level and the epoxidation can be performed at an acceptably low temperature, the longer the catalyst charge can be kept in the reactor and the more product is obtained. Quite modest improvements in the maintenance of selectivity over long periods yields huge dividends in terms of process efficiency.